With slide valves such longitudinal seals are generally disposed above the passage opening and, moreover, rope seals are disposed around these passage openings. Therefore, the medium can not run out between two flat housing parts and the moveable slider plate located between the latter, and so no leakages form. The design of this type of slide valve is described, for example, in is publication EP-A-0 252 367.
Since slide valves constantly need to be designed to be bigger, i.e. nominal passage opening diameters of up to 2000 millimeters or more are required, the problem, in particular with these longitudinal seals, is that the latter are subjected at least partially to higher pressures and contact pressures, on the one hand due to the bending of the housing, and on the other hand due to the wider plates and the resulting higher friction forces. In particular, these longitudinal seals tend to get twisted off, and so they no longer provide sufficient impermeability.